It should be noted that ignition plugs, in particular for gas turbines and reaction motors, can be of two types:                plugs of the high-energy, high-voltage (HEHV) type, whose operating voltage is in the order of 20 kV;        plugs of the high-energy, low-voltage (HELV) type, whose operating voltage is in the order of from 2 to 3 kV;they are capable of supplying energy of from a few tenths of a joule up to several joules.        
HELV plugs comprise a material, such as a cermet, between their electrodes so that the application of an adequate voltage between the electrodes leads to the passage of a spark.
The characteristics of the material during the different phases of the spark has been analysed as follows.
Firstly, the material is active during the ionisation phase which corresponds to an accumulation of charges at the surface of the material. The arcing phase then follows and corresponds to an intermediate zone, during which there is gradual propagation of micro-arcs at the surface. Finally, the sparking phase occurs, during which the material is inactive but is subjected to great mechanical and thermal stresses brought about by the passage of the spark.
It should be noted that, in the common language of ignition plug manufacturers, this material is sometimes referred to as being a “semi-conductor”. However, that usage does not really correspond to the real circumstances because, during the arcing phase, the material does become a conductor at its surface, but not over the entirety of its volume.
The advantages of HELV plugs are, on the one hand, linked with their operation which is not very dependent on the conditions present in the combustion chamber (re-ignition at high pressure), and, on the other hand, linked with the less important ignition train which it is simply necessary to provide in order to ensure their operation. It is those advantages which led to the development of ceramic materials based on silicon carbide, which are described in particular in documents U.S. Pat. No. 5,028,346 and FR-A-2 346 881. Those materials also contain an insulating phase based, for example, on silicon nitride and modified silicon oxynitride, on silicon dioxide, aluminium oxide and alkaline-earth oxide.
However, those systems have not become generally used in motors because the service life of a plug of that type becomes greatly reduced in the event of use under harsh conditions, that is to say, at high pressures and high temperatures combined with chemical attacks linked with the type of fuel used.
In the case of HEHV plugs, it is the wear of the electrodes which limits the service life of the plugs, whilst in the case of HELV plugs the wear of the material placed between the electrodes occurs substantially before the wear of the electrodes, and that is what limits the service life.
The problem addressed by the invention is to increase the service life and therefore the reliability of plugs of the HELV type.
To that end, the invention relates to a method for manufacturing a high-energy, low-voltage ignition plug comprising a ceramic material between its electrodes, characterised in that:                there is mixed, in a container containing a liquid, from 50 to 75% by mass of a compound which is intended to form a conductive phase and from 25 to 50% by mass of one or more materials which allow the formation of phases of yttrium garnet after thermal processing;        operations are carried out for pulverising, drying and sieving the admixture;        the admixture is pressed or injected into a mould;        sintering of the admixture is carried out so as to obtain a ceramic material having porosity of between 0 and 30%;        the ceramic material is used in order to constitute the material located between the electrodes of a high-energy, low-voltage ignition plug.        
The conductive phase is preferably selected from SiC and MoSi2 and admixtures thereof.
The porosity of the ceramic material obtained is preferably between 0 and 15%.
There is preferably added to the admixture a thermoemissive doping agent at a proportion of up to 30% of the total mass of the compounds which are intended to form the conductive and insulating phases.
From 3 to 60% by mass, relative to the total mass of the compounds which are intended to form the conductive and insulating phases, of one or more organic binding and/or plasticising compounds is preferably added to the admixture, and a de-binding operation is carried out after the pressing or injection and before the sintering operation.
It is possible to carry out the pulverising in two steps, the addition of the plasticising agent being carried out between the two steps, and the second pulverising step being less energetic than the first step.
The materials which allow the formation of phases of yttrium garnet may be aluminium oxide and Y2O3.
The yttrium garnet can be an yttrium-aluminium garnet which contains one or more of the compounds Y2O3, Al2O3, YAlO3, Y3Al5O12, Y4Al2O9.
The thermoemissive doping agent can be LaB6.
The invention also relates to an ignition plug of the high-energy, low-voltage type, comprising a ceramic material between its electrodes, characterised in that the ceramic material is of the above type.
As will be appreciated, the invention first consists in using insulating phases of the yttrium garnet type resulting, for example, from the reaction between Al2O3 and Y2O3. They have very high levels of mechanical strength and thermomechanical resistance. Furthermore, their electrical resistivity is very high, which makes them usable in the intended fields of application.
Thus, the element located between the electrodes can withstand the great stresses, to which it is subjected during the ionisation phase (high pressure, high temperature, presence of chemical agents) and also during the sparking phase, in which it is passive but is subjected to thermal and mechanical shocks. Those shocks lead to exposure of the conductive phase particles that are located adjacent to the inter-electrode surface in the case of HELV plugs which use that material and which are known in the prior art.
The invention further allows a substantial reduction in the failure rate of the plugs, which becomes less than 1% in the long term.